1. Field of the Invention
The present invention relates to a substrate for a magnetic recording medium and the method for manufacturing the magnetic recording medium.
2. Description of the Related Art
The recording density (surface density) of magnetic recording has been increased very rapidly, and, in this decade, it has experienced a continuous rapid increase at an annual rate of 50 to 200%. In mass-production level, products having the surface recording density of 70 Gbits/inch2 are commercially available, while in laboratory level, the double surface recording density of 160 Gbits/inch2 has been reported. The mass-production level surface recording density is equivalent to 80 G-bytes per platter for a 3.5″ HDD, and is equivalent to 40 G-bytes per platter for a 2.5″ HDD. In applications of a usual desktop personal computer (a 3.5″ HDD is mounted) and a notebook personal computer (a 2.5″ HDD is mounted), this recording capacity is enough for the applications of mounting a recording medium of only one platter.
The recording density is expected to improve in future. However, a conventional longitudinal magnetic recording mode is reaching a recording limit due to thermal fluctuation. It is considered that when its recording density reaches 100 to 200 Gbits/inch2, a vertical magnetic recording mode will sequentially take over. Although the recording limit of the vertical recording is not known precisely now, it is considered that a value of 1000 Gbits/inch2 (1 Tbits/inch2) may be attained. If such high recording density could be attained, a recording capacity of 600 to 700 Gbytes per platter for a 2.5″ HDD would be attained.
In this regard, in order to realize high density recording, it is required to reduce the flying height of a magnetic recording head from the conventional height of 30 nm to 10 nm or less. Thus, it becomes necessary to smooth the substrate surface. However, it has been found that, if the surface roughness at a micro level (Roughness) is too small, a problem of attachment of the head to the substrate or a problem of decrease of flying stability of the head takes place. Thus, it is ideal to provide a substrate having as small Waviness and Micro-Waviness as possible, as well as Roughness on the order of 0.3 to 2.0 nm. As the terms concerning the roughness of a substrate to be used for a magnetic recording medium having a magnetic film, “Waviness”, “Micro-Waviness” and “Roughness” are used for representing the roughness of observing ranges of 5 to 100 mm, 80 μm to 5 mm, and 80 μm or less, respectively.
In addition, in magnetic recording, if the recording density per unit area is enhanced, the volume of the magnetic recording unit (bit) has to be decreased as a matter of course. However, it is known that, as the volume of a magnetic body originating a ferromagnetic material in charge of recording is decreased, the ferromagnetic material does not continue to be held stable forever, due to a fundamental principle of the magnetism theory. It is known that, as the competition at room temperature between thermal energy kT (k: Boltzmann constant, T: absolute temperature) and anisotropic energy KuV (Ku: anisotropic energy, especially crystal anisotropic energy in case of magnetic recording; V: the volume of unit recording bit) makes the volume of the magnetic recording unit extremely small, or close to the range of kT to KuV, so that the magnetized state of the ferroelectric body becomes unstable. When the unit of magnetization is extremely small like this, the ferromagnetic body behaves as if it is a paramagnetic body. Such a state is called super paramagnetism. It is known that each material has a specific limit size (critical volume) at which super paramagnetism state appears. In an actual magnetic recording, as the volume of recording unit is decreased down to near the critical volume by increasing recording density, the magnetization in a ferromagnetic state decays rapidly as time passes before reaching a super paramagnetic state. Consequently, the magnetization orienting in random directions generates a problem of degenerating magnetic recording information. The occurrence of such a phenomenon in magnetic recording poses extremely serious problems in that recording information written with effort cannot be read out after a certain time passes, or in that writing itself cannot be performed after a certain time passes.
Methods of roughening the surface of a silicon substrate include a dry etching process using chlorine (Japanese Patent Application Unexamined Publication No. 7-263406/1995), and a treatment using alkali hydroxide (Japanese Patent Application Unexamined Publication No. 53-57144/1978), depending on the chemical properties. In these methods, although the surface roughness may be controlled, it is not possible to finish the entire surface of the substrate in a uniform surface roughness (Roughness). Specifically, since the substrate is kept stationary and etched under an uneven circumstance during etching, selective etching occurs so that a flaw appears due to a processing stress remaining in the substrate. As described above, a uniform surface roughness (Roughness) cannot be practically attained by simply using acid or alkali etching. Magnetic anisotropy cannot be obtained in the magnetic recording film by using this treatment alone.